Electronic stethoscope

ABSTRACT

An electronic stethoscope having several modes of operation to process acoustic signals to provide filtered signals useful for diagnosis. In one mode of operation the stethoscope substantially provides only acoustic signals generated by biological activity of the heart. In another mode of operation, the acoustic stethoscope substantially provides only acoustic signals generated by biological activity of the lungs. In another mode of operation, the electronic stethoscope disproportionately amplifies abnormal heart sounds and normal heart sounds to enhance diagnosis of heart abnormalities. The electronic stethoscope is operated in a manner similar to a conventional acoustic stethoscope and has similar spectral characteristics, thus allowing a user with acoustic stethoscope experience to easily use the electronic stethoscope.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/269,128,filed Nov. 8, 2005, which in turn is a continuation of application Ser.No. 11/077,540, filed Mar. 9, 2005, which in turn is a continuation ofapplication Ser. No. 10/872,143, filed Jun. 18, 2004, which in turn is acontinuation of application Ser. No. 10/646,521, filed Aug. 22, 2003,which in turn is a continuation of application Ser. No. 10/313,967,filed Dec. 6, 2002, which in turn is a continuation of application Ser.No. 10/128,149, filed Apr. 23, 2002, which in turn is a continuation ofapplication Ser. No. 09/258,263, filed Feb. 25, 1999, which in turn is acontinuation of application Ser. No. 09/127,992, filed Aug. 3, 1998,which in turn is a continuation of application Ser. No. 08/685,451,filed Jul. 19, 1996, which in turn is a continuation-in-part ofapplication Ser. No. 08/505,601, filed Jul. 21, 1995, entitledELECTRONIC STETHOSCOPE, which prior applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to stethoscopes used fordiagnostic purposes. More particularly, the present invention relates toelectronic stethoscopes and methods for processing signals in electronicstethoscopes for diagnostic purposes.

2. Discussion of the Related Art

Electronic stethoscopes are known in the art. Examples of electronicstethoscopes may be seen in U.S. Pat. Nos. 3,247,324, 4,071,694,4,170,717, 4,254,302, 4,438,772, 4,528,690, 4,534,058, and 4,618,986.

Despite the availability of electronic stethoscopes, they do not appearto be widely used by medical personnel, such as doctors, nurses, andemergency medical technicians. Although the reasons for this lack ofacceptance are not completely clear, one problem with some presentlyavailable electronic stethoscopes may be that they do not reproduceacoustic signals resulting from the operation of various body organs ina manner that is familiar to a trained user. Other problems with somepresently available electronic stethoscopes are that they consume toomuch power, weigh too much, are too large, or require a user to changethe manner in which the stethoscope is used as compared to aconventional acoustic stethoscope.

Medical personnel learn the art of auscultation primarily through theuse of an acoustic stethoscope and are trained to hear normal andabnormal heart and lung sounds based on their specific acousticqualities and their timing relative to other biological sounds. Acousticstethoscopes thus have particular characteristics whose effect upon theacoustic signals heard by the medical personnel become familiar and arerelied upon for diagnosis. Some conventional electronic stethoscopes donot reproduce heart and lung sounds with the same spectralcharacteristics as acoustic stethoscopes.

Therefore, an object of the present invention is to provide anelectronic stethoscope that overcomes at least the above-discusseddisadvantages.

Another object of the present invention is to provide a method forprocessing acoustic signals generated by biological activity to provideenhanced diagnostic information.

SUMMARY OF THE INVENTION

The present invention overcomes the disadvantages of the prior art byproviding an electronic stethoscope that closely resembles a typicalacoustic stethoscope. The present invention feels and operates, to auser, like an acoustic stethoscope, but with enhanced performancecharacteristics.

In one embodiment, the invention includes an electronic stethoscopehaving a first transducer for converting acoustic signals intoelectronic signals, a processing section, having input coupled to anoutput of the first transducer, for processing the electronic signals toprovide selected electronic signals representative of only selected onesof the acoustic signals, and a second transducer, coupled to an outputof the processing section, for converting the selected electronicsignals into acoustic signals.

The electronic stethoscope may include a bandpass filter between thefirst transducer and the processing section to filter out inaudiblesignals. The bandpass filter may also be used to filter out sounds thatare outside the frequency range of heart and lung sounds.

The electronic stethoscope may have several modes of operation. In a“normal” operational mode, the stethoscope transmits the electronicsignals from the first transducer through the processing section to thesecond transducer substantially unchanged. This mode of operation isdesigned to mimic the operation of an acoustic stethoscope so that theacoustic signals heard by the user have substantially the same spectralcharacteristics as they would have if being processed by an acousticstethoscope but with the additional capability of being able to adjustthe volume of the acoustic signals. In this mode, a user can hear, amongother sounds, sounds generated by vascular activity or blood flow.Within this disclosure, the term “normal”, when referring to theoperation of the stethoscope, is meant to refer to the acoustic responseor spectral characteristics of a typical conventional acousticstethoscope but without any response to harmonics outside the frequencyrange of heart and lung sounds. In a “respiratory” operational mode, theelectronic signals from the bandpass filter are filtered by a high passfilter before being transmitted to the second transducer. This filteringoperation substantially filters out electronic signals corresponding toacoustic signals generated by biological activity other than the lungsso that the stethoscope user hears substantially only those soundsgenerated by lung activity. The high pass filter may have a cornerfrequency in the range of 100 to 300 Hz.

The electronic stethoscope may also have a “cardiac” operational mode inwhich the electronic signals from the bandpass filter are filtered by alow pass filter before being transmitted to the second transducer. Thismode of operation substantially filters out electronic signalscorresponding to acoustic signals generated by biological activity otherthan the heart so that the stethoscope user hears substantially onlysounds generated by heart activity. The low pass filter may have acorner frequency in the range of 400 to 600 Hz.

The electronic stethoscope may also have a “murmur enhancement”operational mode in which the electronic signals from the bandpassfilter are processed by an automatic gain control circuit and thentransmitted to the low pass filter. The murmur enhancement mode allowsthe stethoscope to disproportionately amplify, relative to heart soundsgenerated by normal cardiac activity (i.e., for example, so called“dominant” or “first” and “second” heart sounds), heart sounds generatedby abnormal cardiac activity (i.e., for example, murmur sounds) while atthe same time not amplifying the volume of the normal cardiac activity.This allows the user to more clearly determine the relationship betweenthe abnormal heart sound and the normal heart sound. This has the effectof amplifying the low level murmur activity without significantlyamplifying the normal cardiac activity. This mode of operation alsoallows a user to hear heart murmurs more clearly. This mode of operationalso allows a user to hear heart sounds that may be inaudible ordifficult to hear using a typical acoustic stethoscope. In oneembodiment, the automatic gain control circuit has a response timeconstant in the range of 5 to 100 ms.

The various operational modes can be selected in real time without theneed for moving the chestpiece.

In other embodiments, the electronic stethoscope includes a transceiver,coupled between the bandpass filter and the processing system fortransmitting electronic signals from the bandpass filter to a remotedevice and/or receiving electronic signals from a remote device to beprocessed by the processing system. In this way, the electronicstethoscope can transmit the entire spectrum of electronic signals beingdetected to another stethoscope or multiple stethoscopes so that morethan one user can participate in the diagnostic process. In the samemanner the electronic stethoscope can receive electronic signals so thata user can hear these received signals and individually, independently,and simultaneously process the received signals. This allows, amongother things, real-time transmission and reception of the electronicsignals, so that several users can participate in the diagnostic processsimultaneously.

Another feature of the electronic stethoscope of the invention is aswitch for controlling power including a first pole of the switchattached to a first binaural in the pair of binaurals of the stethoscopeand a second pole of the switch attached to a second binaural in thepair of binaurals. A spring is mechanically coupled to each binauraland, in a rest position, urges the first and second binaurals together.When the closing force of the spring is overcome and the binaurals areseparated by a predetermined distance, the first pole and the secondpole make electrical contact to supply electrical power to the signalprocessing circuitry. Upon release of the binaurals, the spring urgesthe binaurals together and the first pole and second pole are separatedto remove electrical power from the signal processing circuitry. Thisprovides a convenient and familiar way of operating the electronicstethoscope that does not require any new activity or steps compared toan acoustic stethoscope that does not have to be turned on or off.

The overall operation of the electronic stethoscope is characterized byfiltering of acoustic signals generated by human biological activity,such as respiratory and cardiac activity, to substantially isolate aselected acoustic signal or set of acoustic signals generated by aparticular organ from the acoustic signals. The electronic stethoscopeperforms this function by converting acoustic signals generated by humanbiological activity into electronic signals, selectively filtering theelectronic signals to provide a filtered electronic signal that containssubstantially only electronic signals representative of acoustic signalsgenerated by a particular organ, and converting the filtered electronicsignal into an audible acoustic signal. In one embodiment, the step ofselectively filtering includes high pass filtering the electronicsignals so that the filtered electronic signal contains substantiallyonly electronic signals representative of acoustic signals generated bylung activity. In another embodiment, the step of selectively filteringincludes the step of low pass filtering the electronic signal so thatthe filtered electronic signal contains substantially only electronicsignals representative of acoustic signals generated by cardiacactivity. In another embodiment, the step of selectively filteringincludes the steps of disproportionately amplifying the electronicsignals and low pass filtering the electronic signals so that thefiltered electronic signal contains substantially only electronicsignals representative of normal cardiac sounds and amplified abnormalcardiac sounds.

Another feature of the invention is the shape of the binaurals thatcarry the acoustic signals to the user's ears. In the present invention,the binaurals are configured so that they conform to the user's body soas to hang comfortably around the user's neck in a “stand-by” position.To accomplish this, each of the first binaural and the second binauralhas a first curve that substantially follows a shape of a human bodyfrom a neck region to a chest region and a second curve thatsubstantially follows a shape of a human body from a base of the neckregion to a shoulder region. Each binaural also has a third curve in theregion of the earpieces and is rotated so that the earpieces aresubstantially aligned with the user's ear canals when the stethoscope isplaced in its “in-use” position.

The electronic stethoscope of the invention may be used to listen tobiological activity (e.g., organ sounds) of humans as well as animals.

The features and advantages of the present invention will be morereadily understood and apparent from the following detailed descriptionof the invention, which should be read in conjunction with theaccompanying drawings, and from the claims which are appended at the endof the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are incorporated herein by reference and whichlike elements have been given like reference characters,

FIG. 1 is an overall view of the electronic stethoscope of theinvention;

FIG. 1A is a side view of the chestpiece of the electronic stethoscopeof the invention;

FIG. 2 is an overall perspective view of the electronic stethoscope ofFIG. 1;

FIG. 3 is a side view of FIG. 1 along line 3-3 illustrating, among otherfeatures, the shape of the binaurals;

FIGS. 4, 5, 6, and 7 illustrate the electronic stethoscope of FIG. 1 inrest and in use positions;

FIG. 8 illustrates a first acoustic topology that may be used in theelectronic stethoscope of FIG. 1;

FIGS. 9 and 9A illustrate a second acoustic topology that may be used inthe electronic stethoscope of FIG. 1;

FIGS. 9B, 9C, and 9D illustrate a third acoustic topology that may beused in the electronic stethoscope of FIG. 1;

FIGS. 10 and 11 illustrate operation of the on/off switch used in theelectronic stethoscope of FIG. 1;

FIGS. 11A and 11B illustrate operation of an alternate embodiment of theon/off switch that may be used in the electronic stethoscope of FIG. 1;

FIG. 12 is a schematic block diagram illustrating how the variousprocessing functions of the electronic stethoscope of FIG. 1 areprovided;

FIGS. 12A and 12B illustrate alternative embodiments for providing themurmur enhancement processing function;

FIG. 12C is an alternate schematic block diagram illustrating how thevarious processing functions of the electronic stethoscope of FIG. 1 areprovided;

FIGS. 13A and 13B illustrate an exemplary circuit implementation of theblock diagram illustrated in FIG. 12; and

FIGS. 14A and 14B illustrate an exemplary circuit implementation of theblock diagram illustrated in FIG. 12C.

DETAILED DESCRIPTION

For purposes of illustration only, and not to limit generality, thepresent invention will now be explained with reference to an electronicstethoscope for use in heart and lung diagnosis in humans. Specificranges of operation and frequencies will be discussed in this context.One skilled in the art will appreciate, however, that the presentinvention is not so limited and that by changing the operationalfrequencies and other stethoscope parameters, the present invention maybe used to diagnose other types of human biological activity as well asbiological activity in infants, children, animals, and so on.

Reference is now made to FIGS. 1, 2, and 3 which Figures illustrate theoverall configuration of the electronic stethoscope of the invention.The electronic stethoscope 10 includes a chestpiece 12 that is used todetect and convert biological activity of particular organs intoacoustic pressure waves (i.e., acoustic signals). The acoustic signalsare transmitted through a flexible acoustic tube 14. An electronicshousing 16 contains circuitry that allows the stethoscope to provide anumber of diagnostic functions, to be described in more detailhereinafter. A knob 18 is located on electronics housing 16 and allows auser of the stethoscope to easily adjust the volume of the acousticsignals produced by the electronic circuitry. Also on the housing arevisual indicators 20, 21, 23, and 25 that illuminate depending on theparticular mode of operation that the stethoscope is in. Visualindicators 20, 21, 23, and 25 may be light emitting diodes. Attached toelectronics housing 16 are first and second flexible acoustic conduits22 and 24. As will be explained in more detail hereinafter, in oneembodiment of the invention, acoustic conduits 22 and 24 transmitacoustic pressure waves from chestpiece 12. In another embodiment of theinvention, acoustic conduits 22 and 24 transmit acoustic signalsresulting from processing by the electronics contained withinelectronics housing 16.

Acoustic conduits 22 and 24 are coupled to a spring and switch housing26. As will be explained in more detail hereinafter, spring and switchhousing 26 contains a switch for controlling the application ofelectrical power to the electronic circuitry in electronics housing 16.

Acoustic conduits 22 and 24 are respectively coupled, through spring andswitch housing 26, to first and second hollow binaurals 28 and 30. Atthe ends 32, 34 of the binaurals are respectively located first andsecond earpieces 36, 38. Earpieces 36 and 38 are typically rubber,plastic, or foam pads used to cushion contact with the user's ears. Thebinaurals 28 and 30 may be aluminum alloy.

Chestpiece 12 has a housing 13 constructed of aluminum alloy resultingin a chestpiece lighter in weight and which we have found to providegreater patient comfort than the stainless steel used in typicalacoustic stethoscopes. The greater patient comfort comes from the factthat the aluminum alloy has high thermal conductivity and the aluminumalloy chestpiece has low thermal mass. Thus, the chestpiece is morelikely to be warmed by the user's hand prior to coming in contact withthe patient's skin, resulting in minimal thermal shock to the patient.The chestpiece uses a diaphragm 15 constructed of polycarbonate. Unliketypical acoustic stethoscopes, the chestpiece in the electronicstethoscope of the invention uses only a single diaphragm and a bell isunnecessary. Adapters or different sized chestpieces tuned to respond todifferent frequency ranges are not required for different patients(e.g., infants vs. adults) because any necessary modification of thespectral characteristics of the stethoscope can be carried outelectronically.

Buttons 40 and 42 on side 37 as well as two other buttons on side 39 areused to switch the electronic stethoscope from one mode of operation toanother. One of the visual indicators 20, 21, 23, or 25 is illuminatedin response to activation of the mode buttons.

A feature of the electronic stethoscope of the present invention is theconfiguration of the binaurals. In many acoustic stethoscopes, thebinaurals lie in a single plane and therefore do not fit the naturalcurves of the body when stored around the user's neck. Many acousticstethoscopes actually pinch the user's neck when stored in this commonstandby position. By contrast, as shown particularly in FIG. 3, thebinaurals of the electronic stethoscope of the invention include a firstcurved portion 44 that is angled so as to lie naturally and comfortablyon the user's neck, shoulders and upper chest with electronics housing16 lying against the user's chest. The binaurals also include a secondcurved portion 46 that allows the binaurals (and the entire stethoscope)to hang comfortably around the user's neck. FIGS. 4, 5, 6, and 7illustrate the electronic stethoscope of the invention in both its “inuse” position (FIG. 4) and its “standby” position (FIGS. 5, 6, and 7).As can be seen from the figures, one of the reasons that the stethoscopefits comfortably around a user's neck is due to the fact that curves 44and 46 are compound and pass through multiple planes. In one embodiment,curve 44 has approximately a radius R1 in the range of 3 to 4 inches andcurve 46 has approximately a radius R2 in the range of 2 to 3 inches. Ina preferred embodiment, radius R1 has approximately a 3.6 inch radiusand radius R2 has approximately a 2.5 inch radius. We have found thatthis combination of radii advantageously provide a stethoscope that fitsa wide range of users comfortably. The shape of the binaurals allows thestethoscope to lie comfortably flat against the user's chest and aroundthe neck. The stethoscope is easily and quickly shifted from its“standby” position to its “in use” position.

An additional curve 45 is also provided having a radius R3 in the rangeof 1.25 to 1.75 inches. In a preferred embodiment, radius R3 hasapproximately a 1.5 inch radius. In addition, the first and secondbinaurals are respectively rotated in the direction of arrows 47, 49 sothat earpieces 36, 38 are angled upwards as shown in FIGS. 3 and 6. Thisallows earpieces 36, 38 to be substantially aligned with the user's earcanals so as to enhance sound transmission in the “in-use” position.

Reference is now made to FIG. 8, which figure illustrates a firstacoustic topology of the electronic stethoscope. The acoustic topologyshould be chosen so as to closely replicate the normal performance of anacoustic stethoscope so as to make the transition from using an acousticstethoscope to the electronic stethoscope intuitive.

In the acoustic topology illustrated in FIG. 8, acoustic signals pickedup by the diaphragm in the chestpiece 12 are routed to the flexibleacoustic tube 14. Flexible acoustic tube 14 encloses two parallelacoustic conduits 50, 52. Acoustic conduit 50 continues throughelectronics housing 16, spring and switch housing 26 and continuesthrough the first binaural 32. The second acoustic conduit 52 continuesthrough electronics housing 16, spring and switch housing 26, andbinaural 34. A microphone 54 is located near the end 34 of the secondbinaural 30. Microphone 54 picks up the sounds transmitted through thechestpiece 12 and acoustic conduit 52 and converts the acoustic signalsinto electronic signals that are processed by the electronic circuitrylocated in the electronics housing 16. A vent 56 is provided at the end32 of binaural 28 to allow excess air pressure to escape that mayotherwise result in distortion caused by excess air pressure on themicrophone. Once the electronic signals have been processed by theelectronic circuitry in electronics housing 16, these signals arereconverted to acoustic signals using miniature headphones 58 and 60located in binaurals 28 and 30, respectively. The first and secondearpieces 36, 38 form a seal with the user's ear canals to block ambientsounds and enhance the transmission of very low frequency sounds throughthe stethoscope. Locating the microphone 54 near the end of the binauralallows the full resonant chamber created by acoustic conduits 50 and 52to enhance the intensity of the low frequencies picked up by thechestpiece before these sounds are converted into electronic signals.

Reference is now made to FIG. 9, which figure illustrates a secondacoustic topology that may be used in the electronic stethoscope of theinvention. In the second acoustic topology, sounds picked up bychestpiece 12 are transmitted through acoustic conduits 62, 64 inflexible acoustic tube 14. As shown in FIG. 9A, the flexible acoustictube 14 encloses two parallel acoustic conduits. This design has beenfound to reduce extraneous noise that could be picked up between thechestpiece and microphone transducer or noise that could be generated bycontact with flexible acoustic tube 14.

Both acoustic conduits 62 and 64 terminate inside electronics housing16. Acoustic conduit 64 is acoustically coupled to microphone 54 insideelectronics housing 16. Microphone 54 may be an electret condensermicrophone. The second acoustical conduit 62 is fully contained withinelectronics housing 16 and terminates with an open end 68. Acousticconduit 62 is of a longer length. The length of acoustic conduit 62 andthe length of acoustic conduit 64 in combination with the open end 68 ofacoustic conduit 62 are selected to provide an acoustic chamber thatimparts to the acoustic signals substantially the same tonal qualitiesas are provided by an acoustic stethoscope. This is advantageous becauseit helps the electronic stethoscope produce acoustic signals that have afamiliar sound to a user. As with the first acoustic topology, the openend 68 of acoustical conduit 62 allows excess air pressure to escape soas to eliminate distortion in the acoustic signals detected bymicrophone 54. The combination of the lengths and the terminationconditions of acoustic conduits 62 and 64 is chosen so as to form aresonant chamber that reproduces the sound characteristics of anacoustic stethoscope.

The microphone 54 picks up the sounds transmitted through the chestpiece12 and flexible acoustic tube 14 and converts the acoustic signals intoelectronic signals. These electronic signals are then processed by theelectronic circuitry contained within electronics housing 16. Theprocessed electronic signals are then reconverted to acoustic signalsusing a single miniature speaker 70. Speaker 70 is acoustically coupledto the first and second flexible acoustic conduits 22 and 24 which aremechanically and acoustically coupled together within electronicshousing 16. The acoustic signals are transmitted through first andsecond acoustic conduits 22 and 24 through first and second binaurals28, 30 and earpieces 36, 38 to the user's ears.

The second topology illustrated in FIG. 9 provides several advantages.First, by mounting the microphone transducer and the speaker inelectronics housing 16, fewer wires need to be run outside of thehousing compared to the first acoustic topology illustrated in FIG. 8.This simplifies manufacturing. Additionally, keeping the microphone andthe speaker inside electronics housing 16 reduces the risk of picking upstray radio frequency noise and of producing unwanted radio frequencyemissions. Also, the second acoustic topology in which a single speakerfeeds binaurals 28 and 30 preserves the conventional earpiececonfiguration of, for example, a typical acoustic stethoscope thusmaking the use of the electronic stethoscope more familiar to users. Inaddition, a single speaker coupled to binaurals 28 and 30 reducesamplification power requirements that in turn reduces the powerconsumption of the electronic circuitry contained in electronics housing16 as well as reducing the number of parts needed. Also using a singlespeaker allows easier control and balance of the acoustic signals in thebinaurals, since the same acoustic signal is provided to both binauralsthrough flexible acoustic conduits 22 and 24.

The acoustic topology illustrated in FIG. 9 also provides severaladvantages with respect to construction and manufacture of thestethoscope. In both the first acoustic topology in FIG. 8 and in anumber of conventional acoustic stethoscopes, the combined acoustic pathcreated by the combination of the chestpiece, tubing, and binaurals isof a fixed length and therefore has a predetermined resonant frequency.The resonant frequency and spectral characteristics are importantfactors in determining the normal response of an acoustic stethoscope.In order to make the overall length from chestpiece to earpiecepractical for a user, and in order to preserve a useable resonantfrequency, the overall length of an acoustic stethoscope for cardiac useis typically approximately 22 to 28 inches. These lengths result inresonant frequencies in the range of 120 Hz-155 Hz.

In the second topology illustrated in FIG. 9, the resonant frequency ofthe combined tube structure of the first and second acoustic conduits62, 64 can be adjusted by increasing or decreasing the length of thesecond acoustic conduit 62 contained within electronics housing 16. As aresult, the overall length of the electronic stethoscope of theinvention can be varied over a relatively wide range (by changing thelength of flexible acoustic tube 14) while maintaining a resonantfrequency within the range of acoustic stethoscopes by increasing ordecreasing the length of the second acoustic conduit (within electronicshousing 16) to compensate for the increase or decrease in the overalllength of the electronic stethoscope. Therefore, a desired resonantfrequency can be held substantially constant over a broad range ofoverall stethoscope lengths. This allows the electronic stethoscope ofthe invention to maintain a resonant frequency comparable to aconventional acoustic stethoscope to provide “normal” soundcharacteristics even if the length of the electronic stethoscope ischanged.

The electronic stethoscope of the invention, as illustrated in FIG. 9,uses an acoustic topology with a tube that has one closed end 65 and oneopen end 68. Sound is introduced near the middle of the tube (with thetotal tube length being the combination of the lengths of acousticconduits 62 and 64) by the chestpiece diaphragm. This topology creates astanding pressure wave with a node at the closed end 65 and an anti-nodeat the open end 68. The wavelength of the a fundamental resonantfrequency of this structure is four times the combined length ofacoustic conduits 62 and 64.

The resonant frequency is approximately given by the following formula:

-   -   where v*1132 ft/sec (speed of sound in air under typical working        conditions for temperature and humidity)        L=overall tube length(ft)

In one embodiment of the invention, acoustical conduit 64 has a lengthof approximately twelve inches, acoustic conduit 62 has a length ofapproximately fifteen inches, and the acoustic path within thechestpiece 12 is approximately one inch. The resonant frequency of thistotal acoustic chamber formed by the chestpiece and the two acousticconduits is approximately 121 Hz. As noted previously, the resonantfrequency of a typical acoustic stethoscope designed for cardiac use isin the range of 120 Hz to 155 Hz. The lengths of the acoustic conduitsin this embodiment have been found to provide a “normal” and familiarsound to trained users of acoustic stethoscopes. The resonant frequencyis within the range of resonant frequencies in typical acousticstethoscopes designed for cardiac use. The acoustic path length can bevaried in order to adjust the resonant frequency and used in combinationwith variations in the parameters of the electronic processing circuitrycontained within electronics housing 16 so as to optimize a particularstethoscope for detection of heart and/or lung sounds for specificapplications such as infants, children, fetuses in the womb, animals ofvarying sizes, and prosthetic heart valves.

Reference is now made to FIGS. 9B, 9C, and 9D, which figures illustratea third acoustic topology of the electronic stethoscope of theinvention. The third acoustic topology is a variant of the topologyillustrated in FIGS. 9 and 9A. In the third acoustic topology, themicrophone 54 is suspended within the flexible acoustic tube 14 inproximity to the chestpiece 12. In one embodiment, the microphone issuspended approximately one inch from the end of the flexible acoustictube 14 that mates to the chestpiece 12. The portion of flexibleacoustic tube 14 containing microphone 54 contains a single acousticconduit that is then split into acoustic conduits 64 and 62 just pastmicrophone 54. Acoustic conduit 64 is sealed or closed at end 65. Themicrophone 54 is suspended preferably concentrically in the tube 14A byan energy dampening foam material 54B as illustrated in cross-section inFIG. 9D. Microphone 54 may be adhesively attached to foam 54B which isin turn adhesively attached to the inside of flexible acoustic tube 14.The energy dampening foam material 54B provides three functionssimultaneously. First, foam material 54B provides air pressure relief byallowing the air column to pass by the microphone and through the foammaterial and into the open acoustic conduit 62. Second, since foammaterial 54B is acoustically transparent to the passband of interest (20Hz to 1600 Hz), the sounds within the passband of interest pass throughthe foam material and into the resonant chamber formed by acousticconduits 62 and 64 so that the sounds detected by microphone 54 have thedesired tonal characteristics. Third, the energy dampening foam 54Bkeeps the microphone mechanically isolated (i.e., decoupled) from theflexible acoustic tube 14 so that any mechanical contact with flexibleacoustic tube 14 does not introduce unwanted noise to the microphone.

A small diameter shielded cable 54A is used to connect the microphone 54to the electronic circuitry in electronics housing 16. This shieldedcable prevents any extraneous radio frequency noises from being pickedup by the microphone circuit. Microphone cable 54A is preferably of asmall enough diameter in order to remain flexible so that bending orflexing of the flexible acoustic tube 14 does not put strain on themicrophone and/or the circuitry within electronics housing 16. As shownin FIG. 9C, shielded cable 54A is embedded within the materialcomprising flexible acoustic tube 14. Alternatively, shielded cable 54Acould be routed through acoustic conduit 62, out through the open end 68and then electrically connected to the circuitry in electronics housing16.

Suspending microphone 54 in tube 14A allows the resonant frequencyproperties of the acoustic topology illustrated in FIGS. 9 and 9A to bepreserved while reducing the extraneous noise that can be introduced tothe microphone through the length of acoustic conduits 62 and 64 in thesecond acoustic topology illustrated in FIGS. 9 and 9A. Soundsintroduced by the diaphragm in the chestpiece 12 can be detected bymicrophone 54 without distortion since large movements of the air inacoustic conduit 14A will travel past the microphone through foam 54Band will be vented through the open end 68 of acoustic conduit 62 ratherthan causing excessive pressure at the microphone.

Also, sounds introduced by the diaphragm of chestpiece 12 will be ableto resonate in the chamber created by acoustic conduits 62 and 64 andthese resonant sounds can be picked up by the microphone to create thenatural and familiar sound produced by acoustic stethoscopes as in thesecond acoustic topology. Thus, low frequency, large intensity soundsfrom diaphragm 15 do not disrupt operation of the electronicstethoscope.

The third acoustic topology provides several additional advantages.First, by placing the microphone near the chestpiece, the microphone hasgreatly reduced sensitivity to noise introduced by the part of theflexible acoustic tube 14 between the microphone and the electronicshousing. Consequently, extraneous noises are not picked up and amplifiedalong with the desired heart and/or lung sounds. Similarly, extraneousnoises produced by anything coming in contact with the flexible acoustictube 14 (such as the user's fingers) do not create unwanted signals thatreach the microphone and are subsequently amplified. This design hasbeen found to reduce, to a greater extent than does the second acoustictopology, extraneous noise that could be picked up between thechestpiece and microphone transducer or noise that could be generated bycontact with flexible acoustic tube 14.

In all other respects, including determination of the resonant frequencyand varying the length of the acoustic conduits to vary the resonantfrequency, the third acoustic topology operates in the same manner asthe second acoustic topology. In one embodiment of the third acoustictopology, acoustic conduits 62 and 64 are both approximately twelveinches long and the acoustic path within the chestpiece 12 isapproximately one inch. The resonant frequency of this total acousticchamber formed by the chestpiece and the two acoustic conduits isapproximately 136 Hz. The lengths of the acoustic conduits in thisembodiment have been found to provide a “normal” and familiar sound totrained users of acoustic stethoscopes. The resonant frequency is withinthe range of typical acoustic stethoscopes designed for cardiac use.

The third acoustic topology may also include vents 26A, 30A in binaurals28, 30, respectively. The vents serve to reduce excess air pressure inbinaurals 28 and 30 generated by transducer 70 and/or by static pressurecreated when the earpieces are closed on the user's ears to therebyreduce any distortion that may reach the user's ears. Vents 26A and 30Acan also be incorporated into the second acoustic topology illustratedin FIGS. 9 and 9A.

Another feature of the electronic stethoscope is the on/off switch 75illustrated in FIGS. 10 and 11. As illustrated in FIGS. 10 and 11,spring and switch housing 26 contains a spring 74 having ends that arerespectively attached to each binaural that applies closing pressure onthe binaural so that binaurals 28 and 30 are continually urged towardseach other in the direction of arrows 76 and 78. Two beryllium coppercontacts 80 and 82 act as switch poles and are incorporated withinspring and switch housing 26. A wire 84 is connected to contact 80 andruns from contact 80 outside binaural 28 and through the first acousticconduit 22 to the circuitry contained within electronics housing 16. Awire 86 is connected to contact 82 and runs outside binaural 30 andthrough acoustic conduit 24 to the electronic circuitry contained withinelectronics housing 16. When the binaurals are pulled apart from theirrest position along the direction of arrows 88 and 90 as shown in FIG.11, in order to be placed in the user's ears, contact points 92 and 94touch, turning on the electronic circuitry contained in electronicshousing 16. When the user removes the stethoscope from his or her earsand spring 74 closes the binaurals together, contacts 92 and 94 areseparated and the circuitry is shut off.

The symmetrical shape of the contacts allows simple fabrication. Inaddition, the right angle shape of contacts 92 and 94 allows precisecontrol over the “turn on” point. Finally, the contact shape allows fora broad range of travel after the turn on point. Once contact points 92and 94 of contacts 80 and 82, respectively, first touch, contacts 80 and82 will simply bow as the binaurals are pushed further apart along thedirection of arrows 88 and 90. Thus, continued separation of thebinaurals along the direction of arrows 88 and 90 does not damage theon/off switch or interrupt the flow of power to the electroniccircuitry.

This switch configuration provides a number of advantages. First, iteliminates the need for timer circuits (that automatically turn theelectronic circuitry off after a predetermined time of nonuse) andmanual on/off switches. Second, it eliminates the need for any standbycurrents that may drain the battery over time. Additionally, the switchand stethoscope are activated through normal usage and do not requireany modification of the user's normal practice when using an acousticstethoscope. Simply spread the binaurals and the stethoscope is turnedon or allow the binaurals to close and the stethoscope is turned off.

In one embodiment of the invention, the switch 75 is activated whenthere is at least four inches of separation between the earpieces 36 and38 of the binaurals. We have found that a four inch separation allowsthe electronic stethoscope to be turned on before being placed on ahuman head, while at the same time insuring that small (for example,accidental) separation of the binaurals will not turn on the stethoscopeand inadvertently drain the battery.

An additional benefit of the spring switch assembly 75 is that itprotects a user from being exposed to a transient signal resulting fromthe application of power to a highly amplified circuit. This transientcould be harmful if earpieces 36 and 38 were already sealed to theuser's ear canals. Since power is applied to the circuits by springswitch 75 before the earpieces reach the user's ears, any transientnoise will have already passed and will not be heard by the user.

The spring 74 may be made of phosphor-bronze or spring steel which hashave the ability to maintain the original shape after bending. Unlike aconventional stethoscope, earpieces 36 and 38 do not need to be sealedas tightly to the user's ear canals because amplification is beingprovided. As a result, spring 74 can have a lower spring constant thansprings of typical acoustic stethoscopes, thus making the electronicstethoscope of the invention more comfortable to use for extendedperiods of time. In one embodiment, spring 74 has a force of 0.5-0.6pounds when the binaurals are separated by 4 to 5 inches.

Reference is now made to FIGS. 11A and 11B which figures illustrate analternate embodiment of the on/off switch 75. As illustrated in FIGS.11A and 11B, the spring and switch housing 26 contains a spring 74having ends that are respectively attached to each binaural that appliesclosing pressure on the binaural so that binaurals 28 and 30 arecontinually urged toward each other in a direction of arrows 76 and 78.

An electrical insulator 74A is disposed between spring 74 and a firstberyllium copper contact 80A. A second beryllium copper contact 82A isdisposed inside housing 26 and spaced away from contact 80A. A separateinsulator 74B may be disposed between contact 82A and spring 74.Alternatively, insulator 74A may extend along the entire length ofspring 74. As with the embodiment of the spring switch illustrated inFIGS. 10 and 11, a wire 84 is connected to contact 80A and a wire 86 isconnected to contact 82A. When the binaurals are pulled apart from theirrest position along the direction of arrows 88 and 90 as shown in FIG.11B, in order to be placed in the user's ears, contact 82A touchescontact 80A, turning on the electronic circuitry contained inelectronics housing 16. When the user removes the stethoscope from hisor her ears and spring 74 closes the binaurals together, contacts 80Aand 82A are separated and the circuitry is shut off.

The embodiment of the spring switch illustrated in FIGS. 11A and 11Balso provides a precise turn on point as in the embodiment illustratedin FIGS. 10 and 11. Once contact 82A has made contact with contact 80A,contact point 94A travels along the length of contact 80A whilemaintaining an electrical connection even if the binaurals are separatedfar beyond their turn on point of approximately four inches separation.Consequently, the spring switch mechanism works consistently andreliably for many different head sizes. Preferably, spring 74 is made ofspring steel because of its ability to maintain its original shape evenafter bending. In all other respects, the spring switch configurationillustrated in FIGS. 11A and 11B provides all of the same advantages asthe switch configuration illustrated in FIGS. 10 and 11.

Reference is now made to FIG. 12 which is a block diagram of thecircuitry contained within electronics housing 16 and which circuitryallows the electronic stethoscope of the invention to carry out a numberof diagnostic functions.

The circuit 98 of FIG. 12 includes a number of sections. An inputsection 100 is used to condition electronic signals. A processingsection 102 processes the electronic signals from the input sectionprovided by microphone 54 according to the particular selecteddiagnostic function. An output section 104 receives the processedsignals from processing section 102 and provides any necessary bufferingand filtering of the signal before the output signal is sent to speaker70. A control section 106 provides control signals for controlling theoperation of processing section 102. Each of the sections will now beexplained in detail.

Input section 100 receives an electronic signal from microphone 54 viabuffer amplifier 110. From buffer amplifier 110, the signal is sent toan input bandpass filter 112. Bandpass filter 112 is an analog filterhaving a passband of between 20 and 1600 Hz. This pass band is thenominal pass band for heart and respiratory sounds. Signals havingfrequencies below 20 Hz are inaudible to the human ear and providingamplification for these sub-audio signals would consume excessiveamplifier power and therefore, signals having a frequency below 20 Hzare filtered out. The output of the bandpass filter 112 is wired to atransceiver interface 114. Transceiver interface 114 includes a normallyclosed switch 116 that under normal conditions, passes the signal frombandpass filter 112 to the input 118 of processing section 102.Transceiver interface 114 provides an interface allowing the electronicstethoscope to send signals to another device, such as a secondelectronic stethoscope, or allows the electronic stethoscope to receivesignals from another device, such as a second electronic stethoscope toallow more than one user to hear and participate in the diagnosis of thesame biological activity. The connection between electronic stethoscopescan be a wired or wireless connection. When a transceiver is pluggedinto transceiver interface 114 and the transceiver is receiving asignal, the signal from chestpiece 12 is disconnected from processingsection 102 to prevent interference. When a transceiver is plugged intotransceiver interface 114 and the transceiver is transmitting a signal,circuitry in the transceiver also routes the signal from chestpiece 12to processing section 102. Transceiver interface 114 also allows asignal that is detected by chestpiece 12 to be recorded for laterdiagnosis. In the same way, a prerecorded signal can be fed intoprocessing section 102 for diagnosis by a user.

A transceiver, such as transceiver 120 may also be used to transmit thesignals detected by the electronic stethoscope to a remote destinationor to receive signals from a remote source. Transceiver 120 may be aninfrared or radio-frequency transceiver. The transceiver may be capableof transmitting only, receiving only, or transmitting and receivingsignals. An infrared transceiver is preferred because infrared signalsdo not cause interference to other radio-frequency devices and are notsubject to radio-frequency interference from other devices. Interferenceis of particular concern in environments such as hospitals where manyradio-frequency devices are used. Since infrared transmission is“line-of-sight”, it does not interfere with devices, for example, inother rooms. The use of a transceiver, such as transceiver 120, allowsthe electronic stethoscope to transmit and receive electronic signalsusing a wireless connection.

The signal from either input bandpass filter 112 or transceiver 120 issent to the input 118 of processing section 102. Processing section 102,under control of control section 106, processes the electronic signalsreceived at input 118 and provides these signals to output 119.

Processing section 102 has four modes of operation. Each mode will beexplained separately.

When the “normal” mode is selected, the acoustic output of theelectronic stethoscope emulates the output of a typical acousticstethoscope. In the normal mode, the processing circuitry providessubstantially flat frequency response between 20 and 1600 Hz whilefiltering out sounds outside of the pass band. In the normal mode ofoperation, the signal from input 118 passes only through selector 122(from line 123) before being transmitted to output section 104 on line124. Input bandpass filter 112 removes harmonic resonances created bythe acoustic tubing of the stethoscope, which tubing can pick upunwanted sounds outside of the desired pass band. As a result, the userhears substantially only the sounds generated by the heart and lungshaving spectral characteristics determined by the acoustic topology ofthe stethoscope.

When the “respiratory” mode is selected, the electronic stethoscopeprovides acoustic signals generated substantially only by the lungs. Inthe respiratory mode, the signal from input 118 is sent along line 125to a fourth order high-pass Butterworth digital filter 126 having acorner frequency at approximately 140 Hz. The nominal pass band fornormal and abnormal human breathing sounds is approximately 140 to 1600Hz. We have determined that the corner frequency of high pass filter 126should be in the range of 100 to 300 Hz. We have found that a cornerfrequency of approximately 140 Hz provides a workable tradeoff betweenthe need to avoid extraneous signals and the need to include signalshaving significant diagnostic information. The output of high passfilter 126 is routed along line 127 through selector 128 along line 130to output section 104. In the respiratory mode, the user hearssubstantially only the acoustic sounds generated by the biologicalactivity of the lungs.

When the “cardiac” mode is selected, the acoustic output of theelectronic stethoscope contains substantially only acoustic signalsgenerated by biological activity of the heart. In the cardiac mode, thesignal passes from input 118 along line 132 through selector 134 andline 136 to a fourth order, low pass, Butterworth digital filter 138.The output of filter 138 is sent along line 140 to processing section104. Low pass filter 138 is set with a corner frequency of approximately480 Hz. The nominal pass band for normal and abnormal heart sounds isbetween approximately 20 and 600 Hz.

We have determined that the corner frequency of low pass filter 138should be in the range of 400 to 600 Hz. We have found that a cornerfrequency of approximately 480 Hz provides a workable tradeoff betweenthe need to avoid extraneous signals and the need to include signalshaving significant diagnostic information. As a result of filtering bylow pass filter 138, in the cardiac mode, the user hears substantiallyonly the sounds generated by biological activity of the heart.

When the “murmur enhancement” mode is selected, the electronicstethoscope disproportionally amplifies acoustic signals generated bynormal and abnormal cardiac activity. In the murmur enhancement mode,the electronic signal at input 118 is sent along line 140 to anautomatic gain control circuit 142. From automatic gain control circuit142, the signal passes through selector 144 along line 146 to low passfilter 138.

In the murmur enhancement mode, both the murmur automatic gain controlcircuit 142 and low pass filter 138 are used so that the user hearssubstantially only enhanced abnormal heart sounds (i.e., heart murmurs)and normal (i.e., so called dominant or first and second) heart sounds.This mode provides disproportionate amplification of heart murmursrelative to dominant heart sounds so as to enhance heart murmurdiagnosis.

Heart murmurs are sounds generated by abnormalities in the heart.Typically, heart murmurs are very low in intensity relative to the firstand second heart sounds. Heart murmurs often can occur within a fewmilliseconds of the beginning or end of the first or second heart sound.In the murmur enhancement mode, the output signal on line 140 includes asignal of the first and second heart sounds having slight amplificationwith low level heart sounds, such as murmurs, being amplified to a levelthat makes them clearly audible compared to the dominant heart sounds.In the murmur enhancement mode, the timing between and among and thefrequency characteristics of the first, second, and abnormal heartsounds is preserved from input 118 to the output of low pass filter 138(and throughout the entire signal path of the electronic stethoscope).This is especially advantageous because the timing of the heart murmurrelative to the first and second heart sounds can be an important factorin diagnosing the heart abnormality. In addition, preserving thefrequency characteristics of the normal and abnormal heart soundsprovides familiar sounds that a user is trained and accustomed to hear.

To accomplish this result, the time constant of the automatic gaincontrol circuit 142 has a relatively short duration. The time constantshould be set so that it is long enough to avoid introducing anynoticeable distortion into the sounds heard by the user.

In addition, the time constant should be short enough so that theautomatic gain control circuit can respond to a rapid change in thevolume level on line 140 resulting from the transition between dominantheart sounds and murmurs. We have found that a time constant having arange of 5 to 100 milliseconds and centered around 10 millisecondsprovides a workable balance between the requirements for the timeconstant. A time constant of 10 milliseconds allows automatic gaincontrol 142 to track normal and murmur heart sounds to provideamplification for each signal so that the output of the automatic gaincontrol circuit 142 on line 146 contains slightly amplified normal heartsounds and clearly audible murmur sounds. The short time constant allowsautomatic gain control circuit 142 to increase the output level of a lowlevel murmur that occurs immediately after a relatively loud dominantheart sound by responding rapidly to the decay in the normal heart soundsignal. Similarly, the relatively short time constant allows theautomatic gain control circuit to rapidly respond to the increasedvolume level of a dominant heart sound which follows a low level murmurand to reduce the gain accordingly so that the relatively loud normalheart sound is not amplified significantly and any resulting distortionis substantially inaudible.

Since murmur sounds may have an extremely low intensity level andautomatic gain control circuit 142 can provide only a fixed maximumgain, not all murmurs can be amplified to a level that is substantiallythe same as the dominant heart sounds. Therefore, automatic gain controlcircuit 142 provides the low level murmur sounds with maximum gain so asto reduce the difference in intensity level between the murmur soundsand the dominant heart sounds. This is what is meant by disproportionateamplification.

Reference is now made to FIGS. 12A and 12B which figures illustratealternative circuits for providing the murmur enhancement function.Circuit 200 illustrated in FIG. 12A uses a variable gain amplifier 202followed by a threshold limiter 204. Variable gain amplifier 202amplifies all of the signals from bandpass filter 112. The amplifiedsignals are then sent to threshold limiter 204 having a presetthreshold. When the dominant heart sounds reach the threshold of thelimiter, they are prohibited from being further amplified. Meanwhile,the gain of the variable gain amplifier 202 is set to increase the levelof the lower level murmur sounds while the output volume of the dominantheart sounds is held constant. Although this circuit performs thedesired murmur enhancement function, when the dominant heart soundsreach the threshold of the threshold limiter and the gain of variablegain amplifier 202 is increased, limiting of the dominant heart soundsmay cause audible distortion.

Circuit 250 illustrated in FIG. 12B uses an analog automatic gaincontrol circuit 252 followed by a logarithmic compressor 254. Automaticgain control circuit 252 provides a constant average output volume levelthat is independent of the level of the input signal. To achieve thedesired transfer response, the nominal volume level, the time constant,and the maximum gain provided in the presence of a low level or no inputsignal are set. As a result, automatic gain control circuit 252normalizes the input signal so that the signal provided to logarithmiccompressor 254 is at a substantially constant level. The time constantof the automatic gain control circuit is set to a duration of severalseconds so that it covers several heartbeats. The output of automaticgain control circuit 252 is therefore a scaled version of the inputsignal. Logarithmic compressor 254 operates in a logarithmic manner tocompress the signal provided by automatic gain control circuit 254 inorder to accentuate the low level signals.

Although the circuit in FIG. 12B operates to perform the desired murmurenhancement function, it also suffers from the same limitations ofcircuit 200 in FIG. 12A. In addition, since a relatively long timeconstant is used to normalize the signal level delivered by theautomatic gain control circuit, it is possible for the user to hear theautomatic gain control circuit working. For example, the user may firsthear the level of the heart sounds at one level while the automatic gaincontrol circuit time constant sets the normalized output level. Once thenormalized output level is set, the volume heard by the user may change.

One skilled in the art will appreciate that the variable gain amplifier202 and the logarithmic compressor 254 could also be used in combinationto perform the murmur enhancement function. One skilled in the art willalso appreciate that the automatic gain control 252 and the thresholdlimiter 204 could also be used in combination to perform the murmurenhancement function.

Although the operation of the murmur enhancement circuit has beenexplained for the

case in which the abnormal heart sounds are at a lower level than thenormal heart sounds, there are occasions when the abnormal heart soundsare actually louder than the normal heart sounds. In this situation, themurmur enhancement circuit operates to amplify the normal heart soundsand provide relatively little amplification for the abnormal sounds.Thus, the circuit can provide amplification for the abnormal heart soundor the normal heart sound, depending upon which sound is of lowerintensity.

One skilled in the art will appreciate that although filter 138 isconnected to the output of automatic gain control circuit 142 in theillustrated embodiment, these devices could be connected so that thesignal is filtered first by filter 138 and then gain controlled byautomatic gain control circuit 142. In the same manner, the circuits ofFIGS. 12A and 12B can be placed before or after filter 138.

When in the murmur enhancement mode, the corner frequency of low passfilter 138 may be the same as the corner frequency when the stethoscopeis operating in the cardiac mode. Alternatively, the corner frequencyfor low pass filter 138 when the stethoscope is operating in the murmurenhancement mode can be different from the corner frequency used in thecardiac mode. The corner frequency for low pass filter 138 can be set atany frequency between the cardiac corner frequency and the overallbandwidth for the electronic stethoscope (approximately 1600 Hz in oneembodiment). For example, if, in the murmur enhancement mode, the cornerfrequency of low pass filter 138 is set to 1600 Hz, high frequencysounds such as those made by prosthetic heart values can be monitored.As explained hereinafter, adjustable clock/oscillator circuit 178 isused to provide a control signal that changes the corner frequency oflow pass filter 138. As a result, the corner frequencies of the filters(for example, filter 126 and filter 138) in processing section 104 canbe set independently for each mode of operation of the electronicstethoscope.

The output at 119 of processing section 102 is provided to resistors 150and 152 which are coupled to the input of a summer 154. The output ofsummer 154 is passed through a second order low pass filter having acorner frequency at approximately 1600 Hz to further limit anyextraneous noise from being passed through the circuit to the user. Fromfilter 156, the signal is passed to the gain control (having a gain setby control knob 18) and to an output volume limiter 158 that includes alimiter having an adjustable threshold that can be pre-set. Thethreshold is set so that sounds introduced into the chestpiece, from,for example, loud voices, banging of the chestpiece on a hard surface orloud ambient noises, cannot exceed a certain level that couldpotentially damage the user's ears. From the output volume limiter, thesignal is amplified by a speaker driver amplifier 160 and then providedto speaker 70.

Processing section 102 is controlled by control section 106. Controlsection 106 includes a number of mode switches, 40-43 coupled to aswitch decoder 170. Each of the modes of operation of the electronicstethoscope is selected by momentarily depressing the corresponding modeswitch. Switch decoder 170 responds to the activation of mode switches40-43 to respectively activate selectors 122, 128, 144, and 134 toprovide the desired mode of operation via control line 172. The controlsignal from switch decoder 170 on control line 172 is also provided to amode indicator control circuit 174 that supplies power to indicators 20,21, 23, and 25, respectively, depending on which mode has been selected.A single indicator is illuminated for each mode of operation. A lowbattery flash signal 176 is also provided to indicator control circuit174 causing mode indicator control circuit 174 to flash the currentlyilluminated indicator when the battery voltage drops below apredetermined level. This signal may also be used to control outputsection 104 to provide an audio signal when the battery voltage dropsbelow the predetermined level. We have chosen a threshold of 1.0 voltsbecause the stethoscope is able to continue operating for several hourswhen the supply voltage reaches this level. This provides a warning tothe user that, although the supply voltage is low, there is sufficientpower available for a few more hours of operation. This type of warningis advantageous because the user is warned before the stethoscopeactually stops operating which is important in, for example, emergencysituations. It also provides a familiar mode of operation, sinceacoustic stethoscopes do not simply stop functioning, thus making theoperation of the electronic stethoscope similar to an acousticstethoscope.

In one mode of operation, the electronic stethoscope defaults to thenormal mode of operation whenever the stethoscope is turned on.Alternatively, if switch decoder 170 is powered directly by the batterythat powers the electronic circuitry, then the electronic stethoscopecan maintain the last mode that was selected before the stethoscope wasturned off by closing of the binaurals. When the power is turned off, avery small current from the battery to switch decoder 170 will keep thelast mode selected by switch decoder 170 active. Therefore, when thestethoscope is turned on the next time, switch decoder 170 will defaultto the last mode selected rather than to the normal mode.

Signal 172 is also provided to an adjustable clock/oscillator circuit178. Circuit 178 provides control signals on line 180 to control thecorner frequencies of the digital filters used in processing section102, such as filters 126 and 138. The use of digital filters inprocessing section 102 allows the corner frequencies of the filters tobe adjusted depending upon the particular application. For example, thedescribed corner frequencies are typically used to detect heart and lungsounds in adults and adolescents. However, we have determined that thefrequencies of the acoustic signals of heart and lung sounds of infantsand children are higher than those of adults or adolescents. As aresult, the corner frequencies of the filters need to be increased. Thisincrease in a corner frequency can be accomplished electronically bychanging the frequency of the clock signal provided by clock/oscillator178 on line 180. A switch (for example 179 in the circuit of FIGS. 13A,13B) in clock/oscillator circuit 178 may be activated to provide a newset of corner frequencies on control line 180 that are appropriate fordetecting heart and lung sounds of infants and children. In all otherrespects, the operation of the circuit is as previously described.

This capability of modifying the corner frequencies of the filters isespecially advantageous. Conventionally, if the heart and lung sounds ininfants and children are desired to be detected, a pediatric acousticstethoscope, generally having a smaller chestpiece and acoustic tubingdesigned to accentuate the higher frequencies is used. Alternatively, anadult acoustic stethoscope may have an adapter attached to thechestpiece designed to be more responsive to the higher frequencies. Thepresent invention eliminates the need for modification of a stethoscopeor the need for a separate stethoscope for infants and children. Oneskilled in the art will appreciate that circuit 178 can be controlled soas to produce a control frequency on line 180 that may be higher orlower than the discussed corner frequencies and that these controlfrequencies can be selected to be appropriate for the particular type ofdiagnosis being performed.

The use of filters having gains which can be set in processing section102 is also advantageous because it allows the signals provided at node119 to be normalized to the signal level in the normal mode on line 124for all modes of operation. By controlling the respective gains offilters 126 and 128 the level of the electronic signal on lines 130 and140 can be adjusted so that it is substantially the same as the signallevel of the electronic signal on line 124. Since the signal level onlines 130 and 140 is substantially the same as the signal level on line124, the output of processing section 102 (and the electronicstethoscope as a result) is substantially the same level without regardto the particular mode of operation chosen. This provides severaladvantages. First, a user does not have to increase the volume manuallywhen switching from a mode with high amplification (for example, themurmur enhancement mode) to a mode with relatively low amplification(for example, the normal mode). In addition, the user is protected fromexcessive transients and amplification when switching from a mode havingrelatively low amplification (for example, the normal mode) to a mode ofoperation have a relatively high amplification (for example, the murmurenhancement mode).

One skilled in the art will appreciate that although filters 112 and 156are analog filters in the illustrated embodiment, these filters could beimplemented using digital technology. One skilled in the art will alsoappreciate that although filters 126 and 138 are digital filters in theillustrated embodiment, these filters could be implemented using analogtechnology.

Reference is now made to FIG. 12C, which figure illustrates an alternateembodiment of the circuitry contained within electronics housing 16 andwhich allows the electronic stethoscope of the invention to carry outthe previously discussed diagnostic functions. In the circuit of FIG.12C, output volume limiter 158 has been eliminated. In addition, asubsonic filter 112A and input limiter 112B process the signal comingfrom band pass filter 112 before it is sent to processing section 102.In all other respects, the operation of the circuit of FIG. 12C is thesame as described in connection with FIG. 12.

The subsonic filter 112A helps to more sharply filter out subsonicsignals (i.e., signals below approximately 20 Hz which are inaudible tothe human ear) that cause distortion and/or consume excess amplifierpower. The corner frequency of subsonic filter 112A is set atapproximately 35 Hz. As discussed previously, in order that theelectronic stethoscope of the invention have the same acousticcharacteristics as a normal acoustic stethoscope, signals belowapproximately 20 Hz should be attenuated as much as possible.Experiments indicate that increasing the corner frequency to much morethan 35 Hz produces a noticeable, audible low frequency rolloff whichcould decrease the ability of the electronic stethoscope to reproducevery low frequency sounds that are still audible. Decreasing the cornerfrequency below 30 Hz produces no appreciable attenuation of signalsbelow 20 Hz unless a higher order filter is used. In one embodiment ofthe invention, subsonic filter 112A is a second order Bessel high passfilter with a corner frequency of 35 Hz. Experiments indicate that thesecond order Bessel high pass filter using a corner frequency of 35 Hzproduces a reasonable tradeoff between the desired attenuation and thecomplications associated with the use of higher order filters that couldbe used to obtain a sharper frequency rolloff with a lower cornerfrequency.

The output of subsonic filter 112A is supplied to input limiter 112B.Input limiter 112B reduces the impact sounds of the user's fingerstouching the chestpiece and/or flexible acoustic tube 14. Input limiter112B limits the magnitude of the input signals supplied to processingsection 102 in a predictable manner so that large sudden transients donot cause noise and/or distortion in the user's ears. The thresholdlevel set for the limiter does not affect normal and abnormal heartand/or lung sounds and these signals pass through the filter stagesunchanged. However, sharp, high intensity impact noises caused by, forexample, the user's fingers moving against the surface of the chestpieceor flexible acoustic tube 14 are substantially reduced by input limiter112B. In one embodiment of the invention, input limiter 112B may use anoperational amplifier with a virtual ground set at 2.5 volts and thelimiter threshold set at 1.5 volts above and below the virtual groundlevel. The limiter is an active circuit, including a diode andoperational amplifier clamping circuit that provides hard limiting atthe 1.5 volt threshold. The gains of input buffer 110, input band passfilter 112, and subsonic filter 112A are set to bring the maximum normalsignal level to approximately 1.5 volts and the stages following thelimiter, i.e. processing section 102 and output section 104, are set tobe responsive to this predictable 1.5 volt limit. Signals that are inexcess of the normal 1.5 volt level are clamped by the limiter circuitat the 1.5 volt level threshold. One advantage of this particularcircuit configuration is that the input limiter provides a fixed limitthreshold regardless of the volume level set in output section 104.

Reference is now made to FIGS. 13A and 13B, which Figures are aschematic diagram of an illustrative circuit embodiment of the blockdiagram of FIG. 12. The illustrated circuit can be powered by a single“AA” alkaline battery and can provide approximately 30 hours ofoperation. If a lithium “AA” battery is used, the circuit can operatefor approximately 90 hours. A battery voltage sensing circuit 180monitors the battery voltage level and controls low battery flashercircuit 182 to flash the then-illuminated indicator to warn that thebattery needs replacement. The integrated circuits used in the circuitof FIGS. 13A and 13B are listed below: Integrated Circuit List (FIGS.13A and 13B) IC# Part # Manufacturer Description U1 LMC662C NationalSemiconductor dual op amp U2 NE578 Philips/Signetics compressor/expander(AGC function) U3 MAX392 Maxim quad switch U4 LMC555C NationalSemiconductor timer U5 LTC1164 Linear Technology switched capacitorprogrammable filter U6 TLE2425C Texas Instruments virtual groundgenerator U7 74HC175 Motorola quad D flip flop U8 MAX777 Maxim dc to dcconverter controller 1 battery cell step-up U9 LMC660C NationalSemiconductor quad op amp U10 LMC660C National Semiconductor quad op ampU11 LMC662C National Semiconductor dual op amp

Reference is now made to FIGS. 14A and 14B, which Figures are aschematic diagram of an illustrative circuit embodiment of the blockdiagram of FIG. 12C. The battery voltage sensing circuit 180 and flashercircuit 182 operate in the same manner as described in connection withFIGS. 13A and 13B. The integrated circuits used in the circuit of FIGS.14A and 14B are listed below: Integrated Circuit List (FIGS. 14A and14B) IC# Part # Manufacturer Description U1A MAX494 Maxim quad op amp U2NE578 Philips/Signetics compressor/expander (AGC function) U3 MAX392Maxim quad switch U4 LMC555C National Semiconductor timer U5 LTC1164Linear Technology switched capacitor programmable filter U6 TLE2425CTexas Instruments virtual ground generator U7 74HC175 Motorola quad Dflip flop U8 MAX777 Maxim dc to dc converter controller 1 battery cellstep-up U9 LMC660C National Semiconductor quad op amp U10 LMC660CNational Semiconductor quad op amp U11 LMC662C National Semiconductordual op amp U12 MAX667 Maxim voltage regulator U13 MAX492 Maxim dual opamp

One skilled in the art will appreciate that although four operationalmodes have been described in detail, the electronic stethoscope can beprovided with additional operational modes.

In the illustrated embodiments, discrete circuitry has been described inorder to select the mode of operation and control of the electronicstethoscope's processing sections. Alternatively, a microcontrollerunder software control could be used. The use of a microcontroller couldallow more than four modes of operation to be selected using theillustrated four mode switches. In addition, the various modes ofoperation could be combined in ways other than as illustrated.Additionally, a microcontroller could be used to select more specificpass bands for specialty use, for example, by cardiologists orpulmonologists, who might want to listen to specific frequency ranges ofthe heart and/or lungs. Furthermore, the microcontroller could be usedto create modes with user-adjustable corner frequencies using, forexample, the volume control 18 to vary the frequency when one of thesemodes is selected or using the mode buttons as controls for up and downsteps of frequency.

A microcontroller could also enable the mode switches to toggle thepediatric mode on and off instead of using a separate switch. In thismode, the microcontroller could also control the lighting of theindicator LED to show the user that pediatric mode had been entered.

A microcontroller could also be used to create a digital volume controlwhich would enable the user to press an up or down button to increase ordecrease the output volume.

In addition, the microcontroller could enable the user to select aparticular reference volume level. The user could then selectivelytoggle between a volume level set by the variable volume control (eitheranalog or digital) in any mode and the reference volume level (userdefinable) for comparison of the sounds heard at different volumelevels. The microcontroller could also generate a reference heart signalfor calibrating and/or setting the reference volume level. Thisreference level could also be useful in establishing standard gradinglevels of murmurs between and among doctors with different levels ofhearing.

Reference is now made to FIG. 15, which figure illustrates a circuitimplementation of a microcontroller and associated circuitry thatprovides the aforementioned features and functions. The circuit 101 ofFIG. 15 enables the electronic stethoscope to have an internally storedreference heart signal that can be listened to by the user to set areference volume level that is stored in a nonvolatile memory 308. Oncethe reference volume level has been selected and stored by the user, theuser can toggle between the user-selected variable volume, level and thereference volume level (in any mode of operation) to compare soundsheard at an amplified or attenuated level against a reference level. Thereference level can be reprogrammed at any time, simply by repeating thecalibration procedure described hereinafter. Circuit 101 also allows thevolume control 302 (either analog or digital) to also provide afrequency control for continuously varying the corner frequencies of thedigital filter in each respective mode of operation.

In one embodiment, microcontroller 300 may be a Microchip 16C73microcontroller. The microcontroller is used as a central control andtiming device and contains an integrated program memory 310, data memory308, and a clock-timer subsystem 312. The microcontroller 300 providesthe interface between the mode switches and the processing section 102.Microcontroller 300 also provides the interface to a pulse generatingencoder (for example, control 302) for volume and/or corner frequencyadjustment. Microcontroller 300 also acts as the clock generator for thecontrol of the corner frequency adjustment of digital filters 126 and138. Microcontroller 300 also controls illumination of the modeindication light emitting diodes 174.

Microcontroller 300 also controls a multiplying digital-to-analogconverter (DAC) 306 to control the output volume level and for playbackof the stored reference heart signal. In one embodiment, the DAC 306 iscoupled between an analog switch 304 and output driver 160. DAC 306 mayby a Maxim MAX504 which features a serial interface to themicrocontroller, low power consumption, and ten-bit resolution.Multiplying DAC 306 scales whatever input level appears at its referenceinput by a digital word loaded from the microcontroller.

When used as a volume control, DAC 306 is loaded with a static value,and analog switch 304 is used to connect the processed stethoscopesignal from low pass filter 156 to the reference input of DAC 306. Whenplaying back the stored reference heart signal waveform, the referenceinput to DAC 306 is switched to a constant DC voltage, and themicrocontroller sequentially loads the DAC with the stored waveform datapoints from memory 308. The microcontroller then loops through thewaveform memory 308, playing back the waveform repeatedly until the modeis changed. The playback volume of the reference heart signal can bevaried by scaling the waveform values in the microcontroller prior toloading them into the DAC 306.

To set a reference volume, a calibration procedure is used in which theuser selects the reference signal mode, for example, by pressing two ofthe mode buttons simultaneously. Once this mode has been selected, theelectronic stethoscope will repeatedly play back the reference heartsignal. The user can then rotate the volume control knob until thedesired volume level of the reference signal can be heard in the user'sears. The reference volume level can be stored by pressing one of themode buttons. The microcontroller 300 then uses the current waveformscaling factor to calculate the static DAC value which is stored innonvolatile memory 308. This value is used to set the user-selectedreference volume when the reference volume mode is selected. The userwill then have the ability, when using the stethoscope to listen to abiological signal, to toggle between any volume setting (as determinedby manipulation of the volume control) and the stored reference volumelevel.

In one embodiment, volume control 302 may be a rotary pulse encoder and,as mentioned previously, the rotary pulse encoder can be used to varythe corner frequencies of the digital filter in the respectiveoperational modes. For example, the rotary pulse encoder can be a BournsECT1D device. As the rotary pulse encoder's shaft is rotated, two pulsestreams that are 90° out of phase with respect to each other areproduced. The order of arrival of the pulses at the microcontroller isused to determine if the shaft is being rotated clockwise (for example,increasing volume or frequency) or counter clockwise (for example,decreasing volume or frequency). The shaft is able to rotatecontinuously, and therefore relative volume or frequency changes arepossible from any shaft position. Microcontroller 300 may use DAC 306 togenerate an audible tone to indicate if extremes of the variable rangeare reached. One skilled in the art will appreciate that themicrocontroller and associated circuitry may also be used in conjunctionwith the circuit illustrated in FIG. 12.

Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications, and improvements willreadily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the spirit and scope of theinvention. Accordingly, the foregoing description is by way of exampleonly and is not intended as limiting. The invention is limited only asdefined in the following claims and the equivalents thereto.

1. An electronic stethoscope, comprising: a first transducer forconverting acoustic signals into electronic signals; a processingsection, having an input coupled to an output of the first transducer,for processing the electronic signals to provide selected electronicsignals representative of only selected ones of the acoustic signals;and a second transducer, coupled to an output of the processing section,for converting the selected electronic signals into acoustic signals.